1. Field of the Invention
The present invention relates to a switching power supply device.
2. Description of the Related Art
Referring to FIG. 5 which shows a constitution of a conventional switching power supply device, the switching power supply device comprises a line filter 101 consisting of coils 101a, 101b each of which is connected in series to each input line of an utility power supply (AC) grounded to a protect earth, a rectifier 102 consisting of four diodes 102a˜102d which are bridge-connected for full-wave rectification of the output of the line filter 101, a smoothing condenser 105 for smoothing the rectified output of a rectifier 102, and a DC/DC converter 106 for converting a voltage across both ends of the smoothing condenser into a predetermined voltage. Condensers (Y condensers) 103, 104 are connected respectively between each output of the line filter 101 and the earth.
In the above-described conventional embodiment, in order to reduce the normal mode noise or common mode noise, the line filter 101 and the condensers 103, 104, which are Y condenser, are used in combination as a countermeasure against noise. In the other case, X condenser is connected between input lines (For example, refer to lines 17 to 25, left column, page 2 and FIG. 3 of Japanese Patent Unexamined Publication JP-A 11-113246 (1999).
In another case, the line filter is connected to a DC line at the output side of the rectifier 102 instead of the input line of the utility power supply (AC). In this case, the line filter is used as a countermeasure against conductive noise while taking resistibility to DC voltage into account, although it is not the inherent use of the line filter.
Such switching power supply device is used in a charger and the like. In view of waterproof and heatproof, filler made of resin and the like is filled within the charger incorporated the switching power supply device in a manner that the filler covers the entire circuit module.
In case where the filler is filled so as to cover the entire circuit module as in the above-described conventional switching power supply device, the effect of waterproof and heatproof is improved. However, the line capacity or impedance of the line filter 101 of the switching power supply device is changed due to the pressure of the filled filler and the like, and hence the frequency characteristic is also changed so that the effect of the line filter 101 is not exhibited as aimed. Therefore, there was a problem that the noise level of the switching power supply device is deteriorated on the contrary due to the filling of filler although the line filter 101 is used.